The invention relates to a reference current source for generating a reference current, comprising:
--a bipolar first transistor and a bipolar second transistor, each having a base, an emitter and a collector, the base of the first transistor being coupled to the base of the second transistor;
--a first resistor connected between the emitter of the first transistor and the emitter of the second transistor;
--a supply terminal;
--a second resistor connected between the emitter of the second transistor and the supply terminal;
--measurement means having inputs coupled to the collector of the first transistor and the collector of the second transistor, and having a measurement output for supplying a measurement signal in response to a difference in the collector current of the first transistor and the second transistor; and
--a bipolar third transistor having a base coupled to the measurement output, having an emitter coupled to the bases of the first and the second transistor, and having a collector for supplying the reference current.
Such a reference current source is known from the IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6, December 1974, A. P. Brokaw "A Simple Three-Terminal IC Bandgap Reference", incorporated herein by reference, pp 388-393, in particular FIGS. 2 and 3. FIG. 2 of the IEEE reference shows a measurement means comprising a differential amplifier having an output connected to the measurement output and having inputs connected to the collectors of the first transistor and the second transistor, a first collector resistor connected between the collector of the first transistor and a further supply terminal, and a second collector resistor connected between the collector of the second transistor and the further supply terminal. FIG. 3 of IEEE reference shows a measurement means comprising a current mirror having an input branch coupled to the collector of the first transistor and having an output branch coupled to the collector of the second transistor and to the measurement output, the input branch and the output branch forming the inputs of the measurement means. In this known reference current source the first and the second transistor operate at different current densities, which is maintained with the aid of the measurement means. The difference between the base-emitter voltages of the first and the second transistor appears across the first resistor as a voltage which is directly proportional to the absolute temperature. As a consequence, the collector currents of the first and the second transistor are also directly proportional to the absolute temperature. The sum of the collector currents flows through the second resistor and generates across this second resistor a voltage which is also directly proportional to the absolute temperature. The voltage on the base of the second transistor is the sum of the base-emitter voltage of the second transistor, which has a negative temperature coefficient and the voltage across the second resistor, which has a positive temperature coefficient. This yields a sum voltage, referred to as the band-gap voltage, whose value is substantially temperature independent over a wide temperature range.
The base-emitter voltage of the second transistor decreases as the saturation current of the second transistor increases. This follows from the well-known relationship between the base-emitter voltage and the collector current of a bipolar transistor. The saturation current of a bipolar transistor is determined by a variety of process parameters which are subject to spread. As a result, the generated band-gap voltage will not have the desired temperature dependence over a specified temperature range and, moreover, the nominal value of the band-gap voltage and hence the nominal value of the reference current derived therefrom will exhibit a spread.